The present invention relates to a reflection type liquid crystal overhead projector to project a display image on a liquid crystal display into an expanded display image.
A conventional reflection type liquid crystal overhead projector comprises, as shown in FIG. 1, a base 11, a mirror Fresnel lens 12 mounted on said base 11, a supporting pole 13 fixed on said base 11, a holding portion 14 mounted in opposition to the base 11 and a light source 15, a projection lens 16 and a mirror 17 mounted on said holding portion 14, a liquid crystal display 18 disposed on said mirror Fresnel lens 12, said liquid crystal display 18 comprising a frame body 21 mounted on a peripheral portion of a liquid crystal panel 19, and a liquid crystal panel driving circuit, interface and other control circuits and a driving unit such as a power supply circuit assembled in the frame body 21. The liquid crystal panel 19 is equipped with polarization plates 23 and 24 on both surfaces of a liquid cell 22.
Light L.sub.1 emitted from the light source 15 is transmitted through the liquid crystal panel 19 and then reflected by the mirror Fresnel lens 12, and the reflected light L.sub.2 is transmitted again through the liquid crystal panel 19 and converged by the projection lens 16 and then reflected by the mirror 17 and projected on a screen 20. Thus, an image on the liquid crystal display 18 is displayed on the screen 20 in an enlarged manner.
A conventional system as described above suffers from a defect that a projected image is shown in double vision. That is, because of the presence of the frame body 21, etc., there inevitably is a gap between the liquid crystal panel 19 and the mirror Fresnel lens 12. Therefore, incident light L.sub.1 emitted from the light source 15 and passed through a pixel Pa in the liquid crystal panel 19 may pass, as reflection light L.sub.2 reflected by the mirror Fresnel lens 12, through a pixel Pb rather than the pixel Pa of the liquid crystal panel 19. Consequently, an image of the pixel Pa may be displayed on the screen 20 in superposition with an image of the pixel Pb.
In order to solve this problem, it has been proposed in the prior art, as shown in Japanese Patent Application Laid Open No. 18339/88 that the liquid crystal panel 19 be closely integrated with the mirror Fresnel lens 12 into one body. However, if such a composition is used, a conventional liquid crystal display is not used and it is not possible to dispose an image film sheet on the mirror Fresnel lens 12 and to display an image on the image film sheet onto the screen in an enlarged manner. In other words, it is not possible to use the equipment in such a manner that an image film sheet and a liquid crystal display can replace each other so that the image on either the film sheet or the liquid crystal display can be displayed in an enlarged manner.
Another problem encountered with a conventional reflection type liquid crystal overhead projector is shown in FIG. 2 where a bright spot 20b appears at a center portion within a panel projection surface 20a on the screen 20, irrespective of the original image, making it difficult to see an enlarged and projected image. As a result of investigating why such a bright spot 20b was created, it was revealed that the image of a filament, etc. of the light source 15 was reflected on the surface of the liquid crystal panel 19, converged by the projection lens 16, projected onto the screen 20 and caused a bright spot 20b.
An object of the present invention is to provide a reflection type liquid crystal overhead projector that can enlarge and display an image on a liquid crystal display without creating a double vision, and without integrating the liquid crystal panel with the mirror Fresnel lens.
Another object of the present invention is to offer a reflection type liquid crystal overhead projector that provides a projection plane with uniform illuminance, without creating any bright spot on a projection surface of the screen resulting from reflection image coming from a surface of the liquid crystal panel and the light source.